Interactions between osmoregulation and the alternative respiratory pathway in Plantago coronopus as affected by salinity

Abstract

Plantago coronopus L., a species from the coastal zone, was grown in culture solution with and without 50 mM NaCl. In addition it was transferred from a non‐saline solution to a solution containing 50 mM NaCl. Short term effects of NaCl on growth and various aspects of energy metabolism, including photosynthesis, shoot dark respiration, root respiration and the contribution of the SHAM‐sensitive alternative pathway to root respiration were investigated. The concentrations of soluble and insoluble non‐structural carbohydrates and of sorbitol a compatible osmotic solute in Plantago, in both shoots and roots were also determined.Growth of shoots and roots was largely unaffected by addition of 50 mM NaCl. Net photosynthesis, shoot dark respiration and the concentration of non‐structural carbohydrates in both shoots and roots were also unaffected by salinity. The rate of root respiration immediately decreased upon addition of 50 mM NaCl. This decrease was almost exclusively attributed to a decreased activity of the SHAM‐sensitive alternative pathway. The concentration of sorbitol in the roots increased quickly after addition of 50 mM NaCl, whilst the increase in sorbitol concentration in the shoots started later. The time course of the increase of sorbitol concentration was similar to that of the decrease in activity of the alternative pathway. During the first 12 h after exposure to 50 mM NaCl, the amount of carbohydrates which was saved in respiration, due to the decreased activity of the alternative pathway, was the same as that used for sorbitol synthesis in the roots. It is concluded that the activity of the alternative pathway decreased due to increased utilization of carbohydrates for sorbitol synthesis, according to a proposed ‘energy overflow model’.After 24 h, the sorbitol concentration in the cytoplasm of the root cells of plants transferred to a saline solution reached a level that was sufficient to compensate for 50 mM NaCl, assuming a cytoplasmic volume of ca. 10% of the total cell volume. The sorbitol concentration in roots of plants grown in a saline environment for several weeks was lower than that in roots of plants transferred to a saline environment for c. 24 h. It is suggested that sorbitol accumulated in roots of Plantago coronopus as an immediate reaction upon salinity, whilst other adaptations may occur thereafter.